1. Field of Invention
The invention relates to processes of making large size full color images by electrographic means. In particular it relates to a multicolor electrographic process using a one-pass printer followed by transfer of the image to a receptor surface.
2. Background of the Art
A general discussion of color electrophotography is presented in "Electrophotography", by R. M. Schaffert, Focal Press, London & New York, 1975, pages 178-190.
Full color reproductions by electrophotography were disclosed by C. F. Carlson in his early patents (U.S. Pat. No. 2,297,691) but no detailed mechanisms were described. Another early patent (U.S. Pat. No. 2,752,833) by C. W. Jacob discloses a method based on a single transparent drum coated with a photoconductor around which a web of receptor paper is fed. Electrostatic images are produced on the drum and by induction on the receptor paper, by three colored line scan exposures from inside the drum using a CRT. Charging stations precede and toner stations follow each of these scan positions with suitable time delays between the scans. The final tricolor image is assembled directly on the imaging paper. In U.S. Pat. No. 4,033,688 (Agfa-Gevaert) a single photoconductive drum is exposed to three different color beams reflected from a color original. The incident reflections occur at points around its circumference, each point being provided with the requisite charging and toning stations. Mechanical time delays provide registration of the three color images which are then transferred to a receptor sheet. Other similar systems are disclosed in U.S. Pat. Nos. 4,403,848 and 4,467,334. All these systems use optical exposure as the method of addressing the imaging surface which avoids mechanical contact with the surface. The use of a sequence of exposure/toning stations immediately following one another as opposed to multiple drum rotations as found in other methods (e.g., U.S. Pat. No. 4,728,983) gives higher production rates for the color prints.
Many patents (e.g., U.S. Pat. Nos. 2,986,466; 3,690,756; 4,370,047) use three or four different photoconductor drums or belts for the different colors and assemble the toned images in register on a receptor sheet.
Exposure by conventional optical scanning is disclosed in many patents e.g., U.S. Pat. Nos. 3,690,756; 4,033,688; 4,234,250. CRT scanning is disclosed in U.S. Pat. No. 2,752,833, and laser scanning on its own or in combination with conventional exposures occurs in patents such as U.S. Pat. Nos. 4,234,250; 4,236,809; 4,336,994; 4,348,100; 4,370,047; 4,403,848; and 4,467,334.
The use of electrographic processes, as opposed to the electrophotographic processes described above, is well represented in the art. In these processes the electrostatic latent image is produced directly by "spraying" charge onto an accepting dielectric surface in an imagewise manner. Styli are often used to create these charge patterns and are arranged in linear arrays across the width of the moving dielectric surface. These processes and the required apparatus are disclosed for example in U.S. Pat. Nos. 4,007,489, 4,569,584, 4,731,542 and 4,808,832. In U.S. Pat. No. 4,569,584 only one stylus array is used and the accepting surface web is traversed to and fro to make the successive images, the toning stations being disposed on either side of the single charging station. In the other three references noted above, the printer comprises three or more printing stations in sequence, each containing both charging arrays and toning stations. In all of these, the multicolor toner image is assembled on the accepting surface and fixed there for display on that surface as a support. None of these references discloses or discusses transferring the assembled image to a receptor surface.
The toners disclosed by C. F. Carlson (U.S. Pat. No. 2,297,691) were dry powders. Staughan (U.S. Pat. No. 2,899,335) and Metcalfe & Wright (U.S. Pat. No. 2,907,674) pointed out that dry toners had many limitations as far as image quality is concerned, especially when used for superimposed color images. They recommended the use of liquid toners for this purpose. These toners comprised a carrier liquid which was of high resistivity e.g., 10.sup.9 ohm.cm or more, and had both colorant particles dispersed in the liquid and preferably an additive intended to enhance the charge carried by the colorant particles. Matkan (U.S. Pat. No. 3,337,340) disclosed that a toner deposited first may be sufficiently conductive to interfere with a succeeding charging step; he claimed the use of insulative resins (resistivity greater than 10.sup.10 ohm.cm) of low dielectric constant (less than 3.5) to cover each colorant particle.
In U.S. Pat. No. 4,155,862 the charge per unit mass of the toner was related to difficulties experienced in the art in superposing several layers of different colored toners. This latter problem was approached in a different way in U.S. Pat. No. 4,275,136 where adhesion of one toner layer to another was enhanced by an aluminum or zinc hydroxide additive on the surface of the toner particles.
Liquid toners which provide developed images which rapidly self-fix to a smooth surface at room temperature after removal of the carrier liquid are disclosed in U.S. Pat. Nos. 4,480,022 and 4,507,377. These toner images are said to have higher adhesion to the substrate and to be less liable to crack. No disclosure is made of their use in multicolor image assemblies.
A number of methods have been disclosed in the patent literature intended to effect liquid toner image transfer with high quality.
The use of silicones and polymers containing silicones as mould release layers and leveling compounds as additives to layers to give release properties is well known.
In the electrophotographic field, photoconductive layers topcoated with silicone layers are disclosed in U.S. Pat. Nos. 3,185,777; 3,476,659; 3,607,258; 3,652,319; 3,716,360; 3,839,032; 3,847,642; 3,851,964; 3,939,085; 4,134,763; 4,216,283; and Jap. App. 81699/65.
In U.S. Pat. No. 3,652,319, easily liquidified solids such as silicone waxes with melting points between 20.degree. C. and 95.degree. C. are applied continually to the photoconductor surface while in use under repetitive cycling conditions. The temperature is slightly elevated at the point of application of the wax to melt and allow spreading of it. Later in the cycle, the wax solidifies into a layer before exposure. The wax layer is renewed every cycle by further applications. The thickness of the wax layer appears to be in the range of 50 nm to 1500 nm with an optimum range of about 200 nm to 800 nm.
U.S. Pat. No. 3,839,032 and its two divisional applications U.S. Pat. Nos. 3,851,964 and 3,939,085 are concerned with liquid toner development and toner image transfer from photoconductors to receptors in which the toner image is temporarily tacky and exhibits more adhesion for the receptor surface than for the photoconductor surface. Novel liquid toner formulations are disclosed having these properties. Low adhesion to the photoconductor surface may be obtained by methods including coating a layer of silicone on the surface. The examples disclose formulations for these layers but give no idea of thickness. Two dependent claims talk of ". . . decreasing the affinity of the photoconductive layer for the tacky image . . . " Introductory discussion indicates the invention (Col. 2 lines 1-16) solves problems of incomplete transfer of liquid toner images and loss of definition experienced in the art.
U.S. Pat. No. 3,850,829 is a later patent and refers to the results in U.S. Pat. No. 3,839,032 as still exhibiting loss of definition. This patent discloses that inclusion of a silicone in the tacky liquid toner gives better results than the silicone layer on the photoconductor.
In U.S. Pat. No. 3,847,642 a transfer film of between 2 .mu.m and 25 .mu.m (preferably about 5 .mu.m) is applied to the photoconductor surface during the imaging cycle. The material must have a low, sharp melting point so that after toning, application of heat melts it and on image transfer part of the layer transfers with the toner and solidifies again. Silicone waxes of low melting point are amongst materials suggested.
In U.S. Pat. No. 4,216,283 one embodiment (Col. 8 lines 63-68, and Col. 9 lines 1-30) describes a thin release layer, which can be of the type of the Syl-OFF.TM. materials, applied to a zinc oxide photoconductor layer (or others which appear to include organic photoconductors) to ensure transfer of the liquid toned image. No indication is given of the thickness of the Syl-OFF.TM. layer or of its relationship with the effectiveness of toner release. The main embodiments and claims concern the use of an abherent layer (e.g. Syl-OFF.TM.) coated intermediate transfer sheets for use with Xerographic system.
In addition to patents dealing with silicone release layers, there are also patents describing the use of silicones in other ways. U.S. Pat. Nos. 3,476,659; 3,594,161; 3,851,964; 3,935,154; and 4,078,927 all disclose the use of silicones as additives to the photoconductor layer itself to give release properties towards both toners and inks (electrographic printing plates). Patents also deal with transfer intermediate sheets, belts, rollers and blankets for transfer of the toned image from the photoconductor to the receptor, in which silicone treatment of the intermediate is proposed. Example patents are U.S. Pat. Nos. 3,554,836; 3,993,825; 4,007,041; 4,066,802; and 4,259,422.
U.S. Pat. No. 4,656,087 discloses dielectric layers for electrographic imaging wherein polysiloxane materials are added to the dielectric resin(s) at the same time as the particulate matter. Japanese unexamined patent application JP 57-171339 published on Oct. 21, 1982 discloses a dielectric layer comprising an organic silicon polymer containing siloxane bonding as the main chain, and another resin in the ratio range 1:4 to 4:1 by weight.
U.S. Pat. No. 4,772,526 discloses photoconductive layer assemblies for electrophotographic systems in which the top layer, either the charge transport layer or the change generation layer, comprises a block copolymer of a fluorinated polyether and a polyester or a polycarbonate. The surface exhibits good toner release properties because of the presence of the fluorinated polyether.
Receptor sheets for the transfer of deposited liquid toner images are well known in the art. For example U.S. Pat. No. 4,337,303 discloses receptor layers which under elevated temperature encapsulate the toner from an imaging surface pressed against the receptor. The physical properties required of the receptor surface are disclosed.